Fine structure in sunspots - III. Penumbral grains

The properties of penumbral grains (PGs) in a medium-size sunspot are studi ed from a 4.5 hour observation series acquired on 5 June 1993 at the Swedis h Vacuum Solar Telescope. La Palma. The application of an image segmentatio n procedure and a feature tracking algorithm on a movie of 360 frames yield s proper motions, intensities, and lifetimes for a set of 469 PGs. Almost 3 /4 of the PGs move toward the umbra and more than 1/4 toward the photospher e. There appears to be a dividing line (DL) in the penumbral approximately 0.7 of the distance from the umbra to the photosphere, such that most PGs o utside this line move toward the photosphere, and those inside move toward the umbra. For inward moving PGs we find a typical proper motion speed of 0 .4 km s(-1) and a median lifetime of 29 minutes, for outward moving ones 0. 5 km s(-1) and 22 minutes. The average speed of inward moving PGs increases with distance from the umbra with a maximum near the DL. Outward moving PG s have maximum speed near the outer penumbral boundary. The measured instan taneous velocities of individual PGs show only partial agreement with theor etical model predictions. We find much shorter lifetimes than earlier autho rs, and no pronounced dependence of lifetime on position in the penumbra. W e discuss possible reasons for the disagreement with previous results.